An extension to the bloc state management library which automatically persists and restores bloc states.

Overview #

hydrated_bloc exports a HydratedStorage interface which means it can work with any storage provider. Out of the box, it comes with its own implementation: HydratedBlocStorage.

HydratedBlocStorage is built on top of path_provider for a platform-agnostic storage layer. The out-of-the-box storage implementation reads/writes to file using the toJson/fromJson methods on HydratedBloc and should perform very well for most use-cases (performance reports coming soon). HydratedBlocStorage is supported for desktop (example).

In addition, while the HydratedBlocStorage client doesn't automatically encrypt/decrypt the data, it is fairly straightforward to implement a custom HydratedStorage client which does support encryption.

Usage #

1. Use HydratedBlocDelegate

void main() async {
  // https://github.com/flutter/flutter/pull/38464
  // Changes in Flutter v1.9.4 require you to call WidgetsFlutterBinding.ensureInitialized()
  // before using any plugins if the code is executed before runApp.
  // As a result, you may have to uncomment the following line if you're using Flutter >=1.9.4.
  //
  // WidgetsFlutterBinding.ensureInitialized();
  BlocSupervisor.delegate = await HydratedBlocDelegate.build();
  runApp(App());
}

2. Extend HydratedBloc and override initialState, fromJson and toJson methods

class CounterBloc extends HydratedBloc<CounterEvent, CounterState> {
  // Use previously cached initialState if it's available
  @override
  CounterState get initialState {
    return super.initialState ?? CounterState(0);
  }

  // Called when trying to read cached state from storage.
  // Be sure to handle any exceptions that can occur and return null
  // to indicate that there is no cached data.
  @override
  CounterState fromJson(Map<String, dynamic> source) {
    try {
      return CounterState(source['value'] as int);
    } catch (_) {
      return null;
    }
  }

  // Called on each state change (transition)
  // If it returns null, then no cache updates will occur.
  // Otherwise, the returned value will be cached.
  @override
  Map<String, int> toJson(CounterState state) {
    try {
      return { 'value': state.value };
    } catch (_) {
      return null;
    }
  }

  @override
  Stream<CounterState> mapEventToState(CounterEvent event) async* {
    switch (event) {
      case CounterEvent.decrement:
        yield CounterState(state.value - 1);
        break;
      case CounterEvent.increment:
        yield CounterState(state.value + 1);
        break;
    }
  }
}

enum CounterEvent { increment, decrement }

class CounterState {
  int value;

  CounterState(this.value);
}

Now our CounterBloc is a HydratedBloc and will automatically persist its state. We can increment the counter value, hot restart, kill the app, etc... and our CounterBloc will always retain its state.

Custom Storage Directory #

By default, all data is written to temporary storage which means it can be wiped by the operating system at any point in time.

An optional storageDirectory can be provided to override the default temporary storage directory:

BlocSupervisor.delegate = await HydratedBlocDelegate.build(
  storageDirectory: await getApplicationDocumentsDirectory(),
);

Custom Hydrated Storage #

If the default HydratedBlocStorage doesn't meet your needs, you can always implement a custom HydratedStorage by simply implementing the HydratedStorage interface and initializing HydratedBlocDelegate with the custom HydratedStorage.

// my_hydrated_storage.dart

class MyHydratedStorage implements HydratedStorage {
  @override
  dynamic read(String key) {
    // TODO: implement read
  }

  @override
  Future<void> write(String key, dynamic value) async {
    // TODO: implement write
  }

  @override
  Future<void> delete(String key) async {
    // TODO: implement delete
  }

  @override
  Future<void> clear() async {
    // TODO: implement clear
  }
}

// my_hydrated_bloc_delegate.dart

class MyHydratedBlocDelegate extends HydratedBlocDelegate {
 MyHydratedBlocDelegate() : super(MyHydratedBlocStorage());
}

// main.dart

BlocSupervisor.delegate = MyHydratedBlocDelegate();